Someone already mentioned Vidalia onions, but for people who like a bit of onion but think they're too strong, they might still be worth a try. At least cooked. They are a much more mild flavor even raw. Many people describe them as sweet.
As far as the strongest onion, I always though red/purple onions had the hardest bite and attacked my eyes the most. I've always considered yellow onions, and even white, to be the more mild variants. /shrug
I like onions but they hurt my stomach and they make breath smell terrible, so I try to stay clear of them. Funyuns are where it's at though.